The present invention relates to casting and, more particularly, to investment casting of a metallic material in a mold in a manner that improves filling of mold and core surface features and reduces casting voids.
In the manufacture of components, such as nickel base superalloy turbine blades and vanes, for gas turbine engines, directional solidification investment casting techniques using gas permeable shell molds have been employed in the past to produce single crystal or columnar grain castings having improved mechanical properties at high temperatures encountered in the turbine section of the engine.
In the manufacture of turbine blades and vanes for modern, high thrust gas turbine engines, there has been a continuing demand by gas turbine manufacturers for internally cooled blades and vanes having complex, internal cooling passages including such surface features as pedestals, turbulators, and turning vanes in the passages to control the flow of air through the passages in a manner to provide desired cooling of the blade or vane. These small cast internal passage surface features typically are formed by including a complex ceramic core in the mold cavity in which the melt is cast. The presence of the complex core having small dimensional surface features to form pedestals, turbulators, turning vanes or other internal cast surface features renders filling of the mold cavity about the core with melt more difficult and more prone to inconsistency. Wettable ceramics and increased metallostatic head on the mold have been used in an attempt to improve mold filling and reduce localized voids in such situations.
U.S. Pat. No. 5,592,984 describes a method of casting a metallic material wherein molten metallic material is introduced into a gas permeable shell mold in a casting furnace under an initial relative vacuum and then a gaseous pressure is applied on the molten metallic material cast in the mold while the mold resides in the casting furnace to improve mold filling and reduce localized void regions in the casting. This method has been successful to improve filling of potential void regions located at ceramic core surface features contacting the molten metallic material (i.e. so-called internal void regions at the core surfaces). This method has been less effective in filling of mold surface features contacting the molten metallic material (i.e. so-called external void regions at the mold surfaces).
In one embodiment of the invention, molten metallic material is cast into a mold that is provided with a refractory barrier to gas permeability effective to delay gas pressure equalization between an exterior and interior of the mold wall that forms mold surface features for contacting the molten metallic material. The molten metallic material is cast into the mold residing in a casting chamber under a first pressure. Then, gaseous pressure is provided in the casting chamber that is higher than the first pressure rapidly enough to reduce or eliminate the presence of localized voids in the casting solidified in the mold.
In a particular embodiment of the invention, the mold wall is provided with a substantially gas impermeable refractory glaze barrier layer at a time when the mold contains molten metal such that the mold wall is substantially gas impermeable through its thickness. The barrier layer retards gas pressure equalization between an exterior and interior of the mold wall and thereby improves filling at mold and core surface features contacting the molten material. An illustrative refractory barrier layer includes a glaze that comprises, before glazing, a majority of silica, a minority of alumina and other oxides.
In a particular embodiment of the invention, the first pressure can comprise a subambient pressure (e.g. a relative vacuum) or ambient pressure (e.g. atmospheric pressure). The higher gaseous pressure is subsequently applied to the molten material in the mold by backfilling the casting chamber with a pressurized gas. Preferably, the gaseous pressure comprises a pressurized gas that is substantially nonreactive with the melt, such as an inert gas.
In another particular embodiment of the invention for making a directionally solidified casting such as a columnar grain or single crystal casting, an investment mold having a plurality of mold cavities and a barrier to gas permeability is disposed on a chill member in the casting chamber, molten metallic material is introduced into the mold so that it flows by gravity from a pour cup through a respective passage to each mold cavity to fill the mold cavities and contact the chill member for unidirectional heat removal, and then the higher gaseous pressure is applied to the material cast in the mold rapidly enough after introduction into the mold to reduce localized void regions present in the cast material.
The above advantages of the invention will become more readily apparent from the following detailed description taken with the following drawings.